1. Field of the Invention
The present invention relates to a magnetic recording medium and, more particularly, to a magnetic recording medium which can reduce noise and also increase a reproducing output along with an improvement in S/N ratio, and a production process for the magnetic recording medium. The present invention also relates to a magnetic recording device, typically, a magnetic disk device, for recording and reproducing information, using the magnetic recording medium of the present invention.
2. Description of Related Art
The development of information processing techniques has led to an increasing demand for an increase in the density of magnetic recording devices such as magnetic disk devices used in external storage devices for computers. Specifically, in the reproducing head of the magnetic disk devices, the use of a magnetoresistive head utilizing a magnetoresistor, wherein the electric resistance changes in response to the magnetic field intensity, that is, an MR head, instead of the conventional inductive thin film magnetic head has been proposed in the art. The MR head applies magnetoresistance, that is, the change in electric resistance produced in a magnetic material on application of an external magnetic field, to the reproduction of a signal on a recording medium and has features including a reproduction output margin that is several times larger than that of the conventional inductive thin film magnetic head, a low inductance and a large S/N ratio. Further, the use of an AMR (anisotropic magnetoresistive) head utilizing anisotropic magnetoresistance, a GMR (giant magnetoresistive) head utilizing giant magnetoresistance, and a spin valve GMR head of a practical type, besides the MR head, have also been proposed.
Further, in order to meet the demand for high density recording, a sufficient improvement in properties, to cope with the above MR head, AMR head, or GMR head (including spin valve head) has been demanded of the magnetic recording medium. In particular, in the magnetic recording medium, it is important to reduce the noise level, thus ensuring a high S/N ratio, because a reproducing output is reduced and at the same time the noise is increased to thereby cause a reduction of S/N ratio, when the recording density is increased.
Accordingly, in the prior art magnetic recording medium, there have been made a wide variety of attempts to obtain a high S/N ratio. Typical attempts include, for example, control in the crystal orientation of the underlayer, improvement in the crystal orientation of the magnetic recording layer, lattice matching of the underlayer with the magnetic recording layer, introduction of an interlayer and others.
Further, in the magnetic recording medium using an aluminum substrate, it is conventional to apply an amorphous NiP layer as a crystal orientation-improving layer over the aluminum substrate, because the NiP layer is effective to increase a crystal orientation of the Cr or Cr alloy underlayer to be directly deposited over the aluminum substrate as a function of the control of the composition of the NiP layer. This is because a nonorientation layer, i.e., an amorphous layer, becomes necessary in order to increase an in-plane orientation of the Co alloy such as CoCrPtTa constituting the magnetic recording layer.
The above reason will be further described. When an interlayer consisting of a Cr-based alloy is inserted between the aluminum substrate and the magnetic recording layer to increase an in-plane orientation of the Co alloy such as CoCrPtTa constituting the magnetic recording layer, it is necessary to orient the Cr-based alloy having a bcc (body-centered cubic) structure of the interlayer to Cr (200). To satisfy this requirement, it is essentially the premise that the underlayer such as NiP layer disposed just below the Cr-based alloy interlayer has an amorphous state.
When the Cr-based alloy is oriented in such a manner that a surface of the interlayer made of such alloy has a (200) plane, 21/2 times the lattice spacing in the (200) plane can substantially conform with the lattice spacing in a c-axis direction of the magnetic recording layer such as CoCrPtTa having a hcp (hexagonal closest packing) structure, and, as a result, the magnetic recording layer can grow so that its c-axis extends in a horizontal direction and thus a (110) plane makes a main plane, thereby ensuring an in-plane orientation of the magnetic recording layer.
In addition to the application of the crystal orientation-improving layer described above, a remarkable reduction of the particle size of the crystals constituting the magnetic recording layer, i.e., formation of finely divided magnetic crystals, is also important to obtain an increased S/N ratio. In the prior art magnetic recording medium, to increase the S/N ratio of the medium, to control a composition of the alloy material constituting the magnetic recording layer, additional elements such as Ta or B have been added to the alloy material.
However, since the magnetic recording layer can be epitaxially grown while reflecting a crystal state of the underlying interlayer made of the Cr-based alloy, the above-described prior art methods for increasing an orientation of the magnetic crystals are unable to remarkably reduce the particle size of the crystals of the magnetic recording layer, and thus they cannot to ensure a sufficiently increased S/N ratio.
On the other hand, as is well-know in the art, the currently available magnetic recording medium has a reduced particle size in the order of about 10 nm in the magnetic crystals of the magnetic recording layer. Apparently, such a reduced particle size cannot be easily obtained by the above methods based on control of the composition of the magnetic layer or addition of the additional elements to the magnetic alloy. Namely these methods can apply to the production of the magnetic recording medium only when a remarkably reduced particle size of the magnetic crystals is not required.